Varistors that contain silicon carbide (SiC), selenium (Se), silicon (Si) or zinc oxide (ZnO) as a primary component are used to protect electronic and electrical apparatuses against overvoltage. Those which conain zinc oxide as a primary component have low voltage limits and large voltage-dependent nonlinearity indices. Thus, they are used more often in protecting apparatuses that are composed of a semiconductor and other devices that have small resistance to overcurrent than those made of silicon carbide.
Voltage-dependent nonlinear resistors fabricated by sintering a mix that contains zinc oxide as a primary component and five auxiliary components in elemental or compound form, i.e., a rare earth element; cobalt (Co); at least one of magnesium (Mg) and calcium (Ca); at least one of potassium (K), rubidium (Rb) and cesium (Cs); and chromium (Cr), are known to have good voltage-dependent linearity. However, these types of resistors are not suitable for incorporation in small devices because they have a relatively small resistance to both long and short wave-tail surges and a short service life.
Resistors having good voltage-dependent nonlinear characteristics can also be fabricated by sintering a mix that contains zinc oxide as a primary component and auxiliary components in elemental or compound form, i.e., a rare earth element; cobalt (Co); at least one of potassium (K), rubidium (Rb) and cesium (Cs); and chromium (Cr). However, these resistors are also not suitable for incorporation in compact devices because they also have a relatively small resistance to long wave-tail surges and a short service life.